HIV-1 and other retroviruses occasionally undergo a high rate of G-to-A substitutions, a phenomenon named hypermutation. HIV-1 genomes that fail to express the accessory protein Vif cannot replicate in primary cells or "nonpermissive" cell lines but can replicate in "permissive" cell lines. APOBEC3G, the dominant-acting host restriction factor responsible for the nonpermissive phenotype, is a cytidine deaminase that is packaged into HIV-1 virions in the absence of Vif and deaminates deoxycytidines in minus-strand DNA to deoxyuridines, resulting in massive G-to-A hypermutation and abrogation of viral replication. HIV-1 Vif binds to APOBEC3G and induces its proteosomal degradation in the virus producer cells, suppressing its virion incorporation and restoring viral replication. Human APOBEC3G is degraded in the presence of Vif but simian APOBEC3G is resistant to Vif; using mutational analysis, we and others recently showed that a single amino acid (D128) is responsible for the species specificity of APOBEC3G proteins. The D128 residue either directly interacts with Vif or is involved in conformational changes that occur upon Vif binding. To gain insights into the mechanism by which APOBEC3G inhibits HIV-1 replication, we recently developed a sensitive cytidine deamination assay using scintillation proximity beads. Using this assay, we demonstrated that interactions with viral and nonviral RNAs that are packaged are sufficient for APOBEC3G virion incorporation and that interactions with viral proteins are not essential for virion incorporation. Our future goals are to elucidate the structure and function of APOBEC3G, identify other host proteins that are critical for APOBEC3G-mediated inhibition of HIV-1 replication, define the nature of the Vif-APOBEC3G interactions, and develop agents that interfere with Vif-APOBEC3G interactions as potential antiviral agents.